Journal of Experimental Marine Biology and Ecology 456 (2014) 8–17 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe Echinoid community structure and rates of herbivory and bioerosion on exposed and sheltered reefs Omri Bronstein ⁎,YossiLoya Department of Zoology, The George S. Wise Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel article info abstract Article history: Echinoid–habitat relations are complex and bi-directional. Echinoid community structure is affected by the hab- Received 19 September 2013 itat structural and environmental conditions; while at the same time, echinoids may also act as ‘reef engineers’, Received in revised form 24 January 2014 able to alter marine environments on a wide geographic scale. In particular, echinoids play a major role in Accepted 9 March 2014 bioerosion and herbivory on coral reefs. Through feeding, echinoids reduce algal cover, enabling settlement Available online xxxx and coral growth. However, at the same time, they also remove large parts of the reef hard substrata, gradually Keywords: leading to reef degradation. Here, we compared coral and macroalgal abundance, echinoid community structure fi Bioerosion and species-speci c rates of echinoid herbivory and bioerosion on reefs subjected to different intensities of oce- Coral reefs anic exposure. Spatio-temporal variations in coral and macroalgal cover were monitored, and populations of the Herbivory four most abundant echinoid species on the coral reefs of Zanzibar – Diadema setosum (Leske), Diadema savignyi MPAs (Michelin), Echinometra mathaei (de Blainville) and Echinothrix diadema (Linnaeus) – were compared between Sea urchins the Island's eastern exposed reefs and western sheltered ones. To account for the effect of management in the Western Indian Ocean context of reef exposure, we included marine protected areas (MPAs) of both types of reef categories (i.e. shel- tered and exposed) in our comparison. Coral and macroalgal cover presented a conspicuous contrasting pattern across exposed and sheltered sites. While coral dominance and lack of macroalgae were prominent on sheltered reefs, an opposite trend of low coral cover and moderate–high macroalgal cover were found on exposed reefs. Bioerosion was also significantly higher on exposed reefs than on sheltered ones (4.2–13 and 1.2–3.9 kg CaCO3 −2 −1 −2 −1 m year , respectively). The highest rates, recorded on Pongwe, with almost 7 kg CaCO3 m year , are among the highest echinoid bioerosion rates known to date. Management had a substantial effect on habitat and echinoid community structure, as coral cover was significantly higher, macroalgal cover lower, and echinoid densities generally reduced on MPAs regardless of exposure intensity. Our findings suggest that exposed reefs are susceptible to markedly higher degrees of echinoid bioerosion; however, adequate management measures can significantly reduce these rates, consequently altering the reef's trajectory for degradation. © 2014 Elsevier B.V. All rights reserved. 1. Introduction 1990; Hawkins and Lewis, 1982). At moderate sea urchin densities this action may facilitate a topographic complexity that favors increased Common coral-reef associated echinoids have a range of different biodiversity (Johnson et al., 2003) and may also enhance coral recruit- feeding modes. Echinoids are considered to be generalist herbivores as ment (Birkeland and Randall, 1981; Carpenter and Edmunds, 2006; their diets may include algae and seaweed (Klumpp et al., 1993; Griffin et al., 2003). However, at high sea urchin densities, echinoids Lawrence, 1975; Vaïtilingon et al., 2003), or omnivores due to the inclu- may limit reef growth through predation of coral tissue (Glynn et al., sion of animal tissue (Briscoe and Sebens, 1988; McClintock et al., 1982), 1979)orextensivecoral(Bak et al., 1984; Mokady et al., 1996) and crus- and even the occasional predation of live coral tissue (Bak and van Eys, tose coralline algae (CCA) erosion (O'leary and McClanahan, 2010). 1975; Carpenter, 1981; Glynn et al., 1979). This dietary flexibility, Moreover, the indiscriminate nature of echinoid grazing has a profound coupled with their great abundance on some coral reefs (Bauer, 1980; effect on coral community composition through its control of newly- McClanahan and Kurtis, 1991), place echinoids as keystone species in settled coral spat (Sammarco, 1980, 1982). Consequently, high sea ur- coral reef environments. As hard-substrate eroders (Bak, 1990; Glynn chin abundance may alter the structure of coral reef communities by et al., 1979; Hunter, 1977; Trudgill et al., 1987) they scrape the surface eroding the reef's coral framework, leading to gradual reef degradation. while grazing (Lawrence and Sammarco, 1982), reducing algal cover Many variables have been recognized as important in regulating (Mapstone et al., 2007) and breaking down reef substratum (Bak, echinoid food consumption. For example, species composition, body size, population densities (Bak, 1990, 1994; Carreiro-Silva and ⁎ Corresponding author. Tel.: +972 3 6409809; fax: +972 3 6727746. McClanahan, 2001; Scoffin et al., 1980), attraction to food (Vadas and E-mail address: [email protected] (O. Bronstein). Elner, 2003), hydrodynamics (Siddon and Witman, 2003), light (Mills http://dx.doi.org/10.1016/j.jembe.2014.03.003 0022-0981/© 2014 Elsevier B.V. All rights reserved. O. Bronstein, Y. Loya / Journal of Experimental Marine Biology and Ecology 456 (2014) 8–17 9 et al., 2000; Vaïtilingon et al., 2003), temperature (Larson et al., 1980), detailed account on echinoid community structure and associated rates and reproductive stage (Klinger et al., 1997), have all been mentioned of herbivory and bioerosion around the Island of Zanzibar, WIO. as factors influencing echinoid feeding rates and ecological impact. How- ever, beyond the physiological aspects determined by the life histories of 2. Methods particular species, echinoid food consumption, and consequently the rates of herbivory and bioerosion, must be considered in terms of the en- 2.1. Study sites vironmental conditions that exist in their habitats, as gradients in the physical environment may produce variability in the abundance and dis- Coral communities and associated echinoid populations were tribution of echinoid populations (Andrew, 1993; Clemente and studied on six reefs surrounding Zanzibar Island (Fig. 1). The sites Hernández, 2008). Several studies have investigated the relationship be- were selected to represent sheltered and exposed reefs in terms of oce- tween coral reef associated echinoids and their habitat (e.g., Dumas et al., anic exposure. To test for effects of marine protected areas, MPAs from 2007; Graham and Nash, 2013; McClanahan, 1998; McClanahan and both exposure categories (i.e. sheltered and exposed) were selected. Kurtis, 1991; O'leary and McClanahan, 2010; Peyrot-Clausade et al., However due to the scarcity of MPAs in the region, only one such site 2000). These publications suggest aspects such as structural complexity, per exposure category was available for this analysis. Three sites, macroalgal and coral cover, sedimentation, and the presence or absence Bawe (06°08.7′S; 039°08.2′E), Changu (06°06.8′S; 039°09.8′E), and of predators, as having substantial effects on the composition, distribu- Chumbe (06°16.3′S; 039°10.2′E), were selected on the sheltered tion, and size of related echinoid populations. For example, marine western side of the main island facing the Zanzibar channel. The site protected areas (MPAs) protecting various echinoid predators conse- at Changu is located ca. 5.5 km from Zanzibar Town and a similar quently present lower rates of sea urchins compared to reefs with distance from the site at Bawe. Chumbe is located ca. 12 km south depauperate predatory populations (McClanahan and Kurtis, 1991; of Zanzibar Town, and has been a private nature reserve, developed McClanahan et al., 1999). Additionally, echinoid communities tend to and managed by the Chumbe Island Coral Park (CHICOP), since display strong differences in species distribution between exposed and 1992 (Nordlund and Walther, 2010). The sites on the exposed eastern sheltered reefs, making sea urchin ecology further complex (Dumas side of Zanzibar were Kiwengwa (06°00.9′S; 039°24.6′E), Pongwe et al., 2007). (06°01.9′S; 039°25.2′E), and Mnemba (05°48.5′S; 039°21.3′E). The Zanzibar Island (Unguja, Tanzania) is situated on the continental shelf of Tanzania between 50°40′ and 60°30′ south of the equator, 35 km from the mainland. Being an island surrounded by coral reefs, ex- N posed to strong easterly winds and with a sheltered west coast, makes Zanzibar an ideal study location for echinoid ecology. Located off the AFRICA East-African shoreline, the island's coral reefs are fundamental to the entire marine environment and of great economic importance for the Mnemba large human population that depends on them for a livelihood (Jiddawi, 1997; Khatib, 1997; Mbije et al., 2002; Ngoile and Horrill, 1993). Small patches of mangrove forest and shallow patches of fringing reefs occur along the more sheltered western coast, while on the more exposed eastern coast fringing reefs slope up to a narrow coastal lagoon backed by sand beaches or fossil coral cliffs (Richmond, 2002). The east- ern and western sides of the island are subject to markedly different wave and current intensities; reefs on the eastern ocean-facing side are exposed to the Indian Ocean (IO) and are susceptible to strong waves and currents, while reefs in the Zanzibar channel, on the Island's Kiwengwa western side, are sheltered from direct exposure to the IO (Bergman and Öhman, 2001; Ngoile, 1990). Swell waves generated in the IO can travel Pongwe undisturbed for thousands of miles before hitting the Island's eastern ZANZIBAR reefs. These swell waves occur off the east coast of Zanzibar for much of the year, changing their orientation from north-east (between Octo- Bawe ber and March) to south-east (between March and October) depending Changu on monsoonal season (McClanahan, 1988b; Zanzibar Department of Environment and MACEMP, 2009).